1. Field of the Invention
The present invention relates to a surface light source device and a liquid crystal display device and, more specifically, to improvement of a surface light source device as a backlight for illuminating, from the back side, a liquid crystal panel which writes data to pixels.
2. Description of the Related Art
Liquid crystal display devices which perform screen display by illuminating a liquid crystal panel with a backlight from the back side has a problem a phenomenon that when a moving picture is displayed images are recognized visually so as to have tails (what is called “moving picture blurring”). To suppress such moving picture blurring, a conventional technique of turning on the backlight in synchronism with data writing to the pixels of the liquid crystal panel was proposed (e.g., JP-A-2001-210122 and JP-A-2003-57622).
In a backlight disclosed in JP-A-2001-210122, a single light guide plate is sectioned into plural blocks and light sources are turned on on a block-by-block basis in accordance with the data writing position of a liquid crystal panel.
A backlight disclosed in JP-A-2003-57622 is equipped with plural light guide plates that are stacked. The light exit area is caused to light divisionally by turning on light sources on a light guide plate basis. This backlight can improve the moving picture characteristics and thereby suppress moving picture blurring because when a moving picture is displayed the light exit area lights divisionally on a block-by-block basis in synchronism with a data writing operation.
JP-T-2003-532273 (The term “JP-T” as used herein means a published Japanese translation of a PCT patent application.) discloses a backlight-related technique for improving the characteristics of light that is output from the light exit area of a light guide plate by detecting light emission quantities of light sources.
As described above, in conventional backlights, to suppress moving picture blurring the light exit area is caused to light divisionally. This results in a problem that the luminance of exit light is prone to be non-uniform among the blocks. This leads to a problem that the manufacturing cost increases if it is attempted to make light that is output from the light exit area uniform among the blocks. In general, in light-emitting elements such as LEDs (light-emitting diodes), the output power is prone to vary with a temperature variation. Therefore, the light emission quantity may vary with an environmental variation. The light emission quantity may also vary with age. Where such light-emitting elements are used for the respective blocks, to make light that is output from the light exit area always uniform among the blocks, it is necessary to detect light emission quantities of light-emitting elements for the respective blocks. To this end, sensors for detecting light emission quantities need to be provided for the respective blocks, which is a factor of increasing the cost of manufacture of a backlight.
The present invention has been made in view of the above circumstances, and an object of the invention is therefore to provide a surface light source device and a liquid crystal display device capable of making light that is output from a light exit area uniform among blocks without increasing the manufacturing cost. In particular, it is an object of the invention is to provide a surface light source device capable of suppressing moving picture blurring in screen display as well as making the luminance of exit light uniform among blocks without the need for providing, block by block, sensors for detecting light emission quantities of light sources on a block-by-block basis.
Another object of the invention is to prevent luminance unevenness and color unevenness in the arrangement direction of point light sources from occurring in a region of a light exit area that is close to the point light sources. In particular, it is an object of the invention is to provide a surface light source device capable of preventing light that is input from each light source to a light guide plate through its end surface from being output directly from a region close to the light source without being diffused sufficiently.